Steel and processing method for the manufacture of high strength, fracture-splittable machinery components

ABSTRACT

The invention relates to a steel and a processing method for high-strength fracture-splittable machine components that are composed of at least two fracture-splittable parts. The steel and method are characterized in that the chemical composition of the steel (expressed in percent by weight) is as follows: 0.40%≦C≦0.60%; 0.20%≦Si≦1.00%; 0.50%≦Mn≦1.50%; 0%≦Cr≦1.00%; 0%≦Ni≦0.50%; 0%≦Mo≦0.20%; 0%≦Nb≦0.050%; 0%≦V≦0.30%; 0%≦Al≦0.05%; 0.005%≦N≦0.020%, the rest being composed of iron and smelting-related impurities and residual matter.

The invention concerns a steel and the processing method for the manufacture of high strength, fracture-splittable machinery components. This material was developed, for example, for the manufacture of crack connecting rods.

The steel must be suitable for forging or for other heat forming processes. The heat used in the forging is dissipated by controlled cooling to produce a largely pearlitic structure that has an apparent limit of elasticity in excess of 750 N/mm², a tensile strength between 1000 and 1200 N/mm², elongation to fracture of over 10% and reduction in area at fracture of over 25%. Its fracture-splittability is a particularly important feature.

The desired properties can be obtained by intentionally creating a pearlitic structure with the precipitation of special carbides (niobium and vanadium carbide) and of manganese sulphides by an appropriately formulated chemical composition, controlled temperature management during the heat forming when producing the preliminary material as well as when forging the finished components (thermomechanical treatment), and a suitable heat treatment after completion of the forging or heat forming, as the case may be.

Up to now, examples of the steels used for this purpose are either mostly eutectoid compositions with approximately 0.7% C, 0.5 to 0.9% Mn, 0.06 to 0.07% S and possibly 0.1 to 0.2% V (C70S6, 70MnVS4), or an average carbon content of approximately 0.4%, about 1% Mn, 0.06 to 0.07% S and about 0.3% V (36MnVS4) according to customers' technical specifications.

These steels have a predominantly pearlitic structure with vanadium carbides and manganese sulphides and conform to requirements regarding mechanical properties. The drawbacks in the various known material alternatives are that the versions used to this point need considerable resources in terms of expensive and scarce alloying materials. Vanadium in particular is currently used increasingly in the field of precipitation hardening ferritic-pearlitic steels (AFP steels), making vanadium an increasingly scarce commodity.

The aim of this invention is to avoid the cited disadvantages by proposing a new steel which has the required mechanical properties regarding strength (and fatigue strength) and, additionally, good toughness characteristics in tensile tests, combined at the same time with good splittability. The steel must also be easy to continuously cast and forge. Furthermore, the new steel must use fewer resources when being produced than the known steels by partially substituting the vanadium content with niobium using an appropriately adapted forming and cooling strategy.

The high value for the apparent limit of elasticity is achieved, other than with the basic composition, by the precipitation of extremely finely dispersed carbides of special carbide formers niobium and vanadium. This requires a solution of the present carbides before the final hot forming process as well as controlled temperature management during the hot forming, followed by a final cooling off stage. Finely dispersed precipitation can be achieved, in particular, by having a low final forming temperature, ending with slightly accelerated cooling. This raises the apparent limit of elasticity in particular, thus improving the elastic limit ratio considerably.

The tensile strength value of the basic composition comprising 0.5% C, 0.6% Si, 1.0% Mn, 0.23% Cr, 0.2% Ni and 0.14% V can be adjusted to the desired level by slightly accelerated cooling after hot forming.

The toughness characteristic values are controlled, in particular, by selective alloying with 0.06 to 0.07% sulphur. The carbon content and relatively high nitrogen content also act positively in this regard.

A crystalline fracture or macroscopic deformation is essential for the material to split properly. This is achieved by designing the alloy with high contents of carbon, nitrogen and sulphur, and comparatively low contents of chromium, nickel and molybdenum.

According to the solution proposed, the invention concerns a steel for the manufacture of fracture-splittable components for the vehicle industry with the following chemical composition in percentages by weight:

0.4%≦C≦0.6%; 0.2%≦Si≦1.0%; 0.5%≦Mn≦1.5%; 0%≦Cr≦1.0%; 0%≦Ni≦0.5%; 0%≦Mo≦0.2%; 0%≦Nb≦0.05%; 0%≦V≦0.3%; 0%≦Al≦0.05%; 0.005%≦N≦0.020%

whereby the remainder consists of iron and impurities from the melting process.

Disadvantages of Old Alternatives?

-   -   Partly the high cost of alloying with 0.29% V (with 36MnVS4)     -   scarcely any alternatives to patented steel grade

Benefits of the new alternatives?

-   -   Low alloying costs (only 0.14% V)     -   Customer not dependent on a single patented steel grade 

1. A steel and the processing method for the manufacture of high strength, fracture-splittable machinery components, wherein they consist of at least two fracture-splittable parts, wherein their chemical composition has the following components in percentages by weight: 0.40%≦C≦0.60% 0.20%≦Si≦1.00% 0.50%≦Mn≦1.50% 0%≦Cr≦1.00% 0%≦Ni≦0.50% 0%≦Mo≦0.20% 0%≦Nb≦0.050% 0%≦V≦0.30% 0%≦Al≦0.05% 0.005%≦N≦0.020% whereby the remainder consists of iron and impurities from the melting process and residues.
 2. A steel in accordance with claim 1, wherein its chemical composition is the same except that: 0.10%≦V≦0.20%
 3. A steel in accordance with claim 1, wherein its chemical composition is the same except that: 0.020%≦Nb≦0.030%
 4. A steel in accordance with claim 1, wherein its chemical composition is the same except that: 0.010%≦N≦0.020%
 5. A steel in accordance with claim 1, wherein its chemical composition is the same except that: 0.45%≦C≦0.55% 0.50%≦Si≦0.70% 0.90%≦Mn≦1.10% 0.10%≦Cr≦0.40% 0.10%≦Ni≦0.30% 0.10%≦V≦0.20% 0.010%≦Al≦0.020% 0.020%≦Nb≦0.030% 0.010%≦N≦0.020%
 6. A steel in accordance with claim 1, wherein its chemical composition is the same except that: 0.45%≦C≦0.55% 0.50%≦Si≦0.70% 0.90%≦Mn≦1.10% 0.10%≦Cr≦0.40% 0.10%≦Ni≦0.30% 0.10%≦V≦0.20% 0.010%≦Al≦0.020% 0.020%≦Nb≦0.030% 0.010%≦N≦0.020% 0.020%≦Ti≦0.030%
 7. A steel in accordance with claim 1, wherein its chemical composition is the same except that: 0.45%≦C≦0.55% 0.50%≦Si≦0.70% 0.90%≦Mn≦1.10% 0.30%≦Cr≦0.40% 0%≦Ni≦0.20% 0.10%≦V≦0.20% 0.010%≦Al≦0.020% 0.020%≦Nb≦0.030% 0.015%≦N≦0.020%
 8. The use of a steel in accordance with claim 1 to manufacture fracture-splittable components used in the construction of vehicles, whereby they have a predominantly pearlitic structure from the precipitation of special carbides after forging and controlled cooling.
 9. A component in accordance with claim 8, wherein the apparent limit of elasticity is over 750 N/mm² after cooling down from the forming temperature.
 10. A component in accordance with claim 9, wherein the tensile strength falls between 950 N/mm² and 1200 N/mm² after cooling down from the forming temperature.
 11. A component in accordance with claim 10, wherein the elongation to fracture exceeds 10% after cooling down from the forming temperature.
 12. A component in accordance with claim 11, wherein the reduction in area at fracture exceeds 25% after cooling down from the forming temperature.
 13. A component in accordance with claim 12, wherein it is fracture splittable.
 14. A component in accordance with claim 13, wherein it is suitable for induction hardening.
 15. A component in accordance with claim 14, wherein the mechanical properties can be adjusted both in the material prior to forging as well as in the component through the use of thermomechanical treatment. 